1. Field of the Invention
The present invention relates to an organic light emitting diode display device, and more particularly, to an organic light emitting diode display device capable of sensing driving current of each pixel with a simple configuration to compensate for a luminance deviation between pixels and a pixel current sensing method thereof.
2. Discussion of the Related Art
An organic light emitting diode (OLED) display device is a self-emission element that makes an organic emission layer emit light according to recombination of electrons and holes. The OLED display device is expected to be a next-generation display device since it is ultra-thin and has high brightness and low driving voltage.
Each pixel constituting the OLED display device includes an OLED composed of an anode, a cathode and an organic emission layer interposed between the anode and the cathode, and a pixel driving circuit that independently drive the OLED. The pixel driving circuit includes a switching thin film transistor (TFT), a capacitor and a driving TFT. The switching TFT charges the capacitor with a voltage corresponding to a data signal in response to a scan pulse. The driving TFT controls emission of the OLED by adjusting current supplied to the OLED according to the level of voltage charged in the capacitor. The emission of the OLED is proportional to current supplied from the driving TFT.
However, in the OLED display device, pixels may have different driving TFT threshold voltages Vth and mobility due to process variation, causing OLED driving current to be different for respective OLEDs, generating a deviation in characteristics of driving TFTs of the pixels. In general, an initial driving TFT characteristic difference generates unevenness or a pattern on a screen and a characteristic difference due to deterioration of driving TFTs, which occurs when the driving TFTs drive OLEDs, reduces the lifespan of an AMOLED display panel or generates a residual image.
To solve these problems, a prior art patent such as U.S. Pat. No. 7,834,825 discloses a method of sensing current of each pixel and compensating for input data according to the sensing result. However, since this technique uses a method of sensing current flowing to a power line (VDD or VSS line) of the panel while turning pixels on, current sensing time increases due to a parasitic capacitor located in parallel with the power line, making high-speed sensing difficult when resolution increases.
Furthermore, although a plurality of current sensing circuits may simultaneously sense currents of a plurality of pixels, this increases circuit scale. Therefore, while the conventional technology can compensate for an initial characteristic deviation among driving TFTs by measuring the initial characteristic deviation during a test process performed before products are shipped, the conventional technology has difficulty in sensing and compensating for characteristic deviation due to deterioration of the driving TFTs, which occurs when OLEDs have been driven after products are shipped.